The production of anhydrides by oxidation of an appropriate hydrocarbon in the presence of a suitable catalyst is well known. The production of maleic anhydride, for example, from a gaseous feed of butane and air is described by M. Malow, "Benzene or Butane for MAN," 59 Hydrocarbon Processing 149-153, (November 1980).
Typically, maleic anhydride is produced by vapor phase oxidation of normal butane (n-butane) over a vanadium-phosphorus oxide catalyst. The reaction produces carbon monoxide, carbon dioxide, water, and smaller amounts of other partially oxidized byproducts in addition to maleic anhydride. The reaction can be carried out either in a fixed, fluidized, or transport bed reactor.
Federal Republic of Germany (FRG) patent application Disclosure No. 25 44 972 ('972) discloses that after maleic anhydride recovery, 75 to 98% of the off-gas is recycled. From the remaining gas, butane is recovered using a two-bed TSA (temperature swing adsorption) process. The TSA is a very energy intensive method to recover butane. Also, this process removes only about 1.5 to 18.4% of the nitrogen in the air feed to the maleic anhydride reactor but not all the CO and CO.sub.2 produced in the maleic anhydride reactor This will result in considerable inert gas accumulation and a reduction in reaction efficiency. Hence, this process requires operation either at a high pressure or with the use of almost pure oxygen to maintain maleic anhydride production as compared to a once-through process.
A process to recover unreacted hydrocarbon by condensation is shown by U.S. Pat. No. 4,352,755. Butane recovery by condensation is an energy intensive method, especially since, after recovery, butane has to be reheated to the reaction temperature. Also, if the feed contains nitrogen, the cooling duty required for condensation will be tremendous.
More recently, Federal Republic of Germany patent application Disclosure No. 35 21 272 ('272) shows a process using pure oxygen and butane as feed to a fixed bed reactor, with the feed concentration of oxygen below the stoichiometric ratio. The '272 patent suggests that the oxidation reactor is operated at a very low butane conversion. The unreacted butane is recovered and recycled from the off-gases following maleic anhydride recovery by compressing and flashing it to remove butane, CO.sub.x, maleic anhydride, and water. It is estimated that the pressure required to achieve the above is very high. Again, this butane recovery scheme is energy intensive and may not be practical when the feed contains nitrogen.
H. Bosch et al., "Selective Oxidation of n-Butane to Maleic Anhydride under Oxygen-Deficient Conditions over V-P-O Mixed Oxides," 31 Applied Catalysis 323-337, at 335 (1987) concludes that operation of the process in oxygen-deficient conditions would result in lower than normal maleic anhydride yields. Since typical oxidation catalysts must always be kept in oxidized form, a lower butane conversion coupled with limited oxygen in the process could result in catalyst deactivation.
U.S. Pat. No. 3,904,652 shows a process where, after maleic anhydride recovery from the reactor effluent gases, off-gases containing unreacted butane, nitrogen, CO.sub.x, and oxygen is recycled to the reactor. About 11% of the off-gas is purged to avoid inert gas (e.g., CO.sub.2, N.sub.2) build up in the process. However, about 7% of fresh butane fed to the reactor is lost in the purge, and the necessary incineration of this purge requires additional fuel.
It is therefore apparent that industry is still searching for a cost effective process of converting hydrocarbons into anhydrides. The process of the present invention is cost effective and the disadvantages of the aforementioned systems are substantially reduced or eliminated therein. Moreover, in comparison to conventional processes, the thermal energy requirements of the present invention are markedly reduced.